I have had good success with 6 Wraithguard in a Tantalus. I bring D scythes, an archon (with a blaster, a portal and a Shadowfield) and a Autauch. The Wraithguard, Tantalus and archon together are expensive and so I bring the Autauch to make sure they can drop in when I need them to. When placed well this unit can take out pretty much anything you need it to. Imperial Knight, no problem. Void shield generator (and multiple units clustered around it), dead! Tank wall, destroyed.
The big issue with this unit is that in order to really get value for the points, you have to kill a lot of things (or the right things). That means killing multiple units a turn. Placement on the turn it drops in must anticipate what will happen in future turns. A crafty opponent will try to limit what you can flame that first turn.
Once your opponent sees the damage that the thing deals out, he will focus almost everything he has to kill the Tantulus and it's passengers. That can be useful too. Venoms come into their own when not being shot at.
The Wraithguard are pretty safe in a 5 hull point vehicle with a flicker field and 3+ cover save. Nobody is going to assault the thing with all those flamers. Watch out for Helldrakes, they won't hurt the Tantalus but will kill the Writhguard. Also, don't jink if there is a chance of being assaulted. You won't be able to overwatch.
I have seen that these days most tournaments will allow the Tantalus. I may end up taking it to LVO next year.
RB